The Orlando Magic, bidding for a National Basketball Association expansion team, can rest assured that it has the full backing of the city of Orlando. The Magic would be the primary tenant in a new downtown arena.

But what are the chances of the Magic becoming a reality? Will the NBA expand? How many teams will it add? When? Can Orlando outbid larger cities? Will looming labor problems and the shaky financial footing of some current teams delay expansion?

There will be no firm answers to those questions until the NBA owners discuss expansion next Monday in Phoenix, Ariz.

''I can remember, four to five years ago, when we were meeting and talking about what four clubs we should close,'' NBA Commissioner David Stern said. ''I'm not so sure that we need to move so quickly now to expand.''

At the NBA's annual summer meetings in Orlando several weeks ago, Stern said that if the NBA did expand it should do so cautiously, perhaps by adding one team for the 1987-88 season.

Such strategy would rule out the Magic, because Orlando's arena will not be ready until the 1988-89 season. It also would ruin the efforts of a group in Miami, whose arena will come on line at about the same time as Orlando's.

Three of the six cities interested in expansion -- Minneapolis; Charlotte, N.C.; and Toronto, Canada -- are prepared to field a team in 1987-88. The others are awaiting construction of arenas. A group in Orange County, Calif., that has applied to the NBA for an expansion team does not have an arena and has yet to organize a season-ticket drive.

''I think what it all boils down to is that the NBA will only expand if it can find a great opportunity,'' said Magic General Manager Pat Williams, former general manager of the Philadelphia 76ers. ''They are going to look at the city that has the best overall package, and I think that when they judge Orlando they'll find us outstanding in every area.''

Speculation throughout the league is that, despite Stern's feelings, the NBA may well add as many as three clubs, with one beginning play in 1987-88 and two others in '88-'89.

The best guess among various league officials contacted by The Orlando Sentinel is that Minneapolis, a proven major-league market for sports, will be granted the first expansion team, followed, perhaps, by Orlando and Miami.

Orlando, considered a long-shot for expansion only a few months ago, now is a favorite after a successful season-ticket reservation campaign. The Magic have sold 11,930 reservations at $100 each, more than any other candidate. Season-ticket reservations are expected to be a key criterion for the owners when they vote.

Nevertheless, there is a chance that the Magic may not become a reality and that none of the other expansion applicants will be granted teams.

There is expected to be squabbling among the owners over whether there should be any expansion at all.

The NBA's collective-bargaining agreement with the players' union will expire after this season, and talks about a new deal are expected to be long and difficult, tempting some owners to delay expansion until the labor problems are resolved. Such potential obstacles have not dulled the Magic's enthusiasm.

''They say an expansion team will cost $25 million,'' Williams said. ''You multiply that by three expansion teams, and you're talking about a lot of money $75 million. Frankly, I don't know if the owners can turn that down.''